This invention relates to bonding apparatus, and more particularly to a method and apparatus for applying an elongate continuous adhesive-coated reinforcing material to the surface of a moving substrate.
In the plywood industry, manufacturing procedures often include adding a continuous strand of a reinforcing material to the surface of a substrate such as a veneer. In one such procedure, a reinforcing string or tape that is coated and impregnated with a hot melt adhesive is applied to the surface of composed veneer sheets. The hot melt adhesive is typically a thermoplastic adhesive which is a solid at lower temperatures and which is softened for application at higher temperatures. For proper application of the adhesive-impregnated string or tape, the adhesive must be heated to its flow point before being applied to the substrate. Thus the application equipment for the adhesive-coated reinforcing material typically includes a heating element for softening and activating the adhesive, and a wheel, often chilled, for pressing and bonding the heated string or tape to the surface of a substrate. Typical application equipment is shown, for example, in U.S. Pat. No. 4,285,758 to Clausen et al., and U.S. Pat. No. 4,692,202 to Getz.
During application, as the adhesive-coated string or tape material is exposed to the heating element, the adhesive is heated to its flow point. Some of the adhesive may flow and separate from the string or tape before the material is applied to the substrate. Thus, when using a conventional applicator, over a period of time the heating element becomes fouled with the separated hot melt adhesive which chars and solidifies in the heating element. When this occurs the manufacturing process must be interrupted while the applicator is shut down, cooled, and the heating element is cleaned. During the cleaning process the operator attempts to remove the solid and charred adhesive without damaging the heating element, for example by drilling, brushing or scraping the adhesive from the heating element. Typically, the methods most effective in removing the adhesive are also the most hazardous to the heating element. Often the entire heating element must be replaced, either because it has become too fouled to be cleaned or because it is damaged during cleaning.
In addition, when the applicator is used to apply the string or tape material to the underside of a substrate, separated adhesive also drips onto the applicator frame or onto the floor as the heated string or tape material leaves the heating element. In this case, frequent cleaning of the dripped adhesive from the frame and floor is required to prevent debris from accumulating at the applicator, and production must often be curtailed for this routine maintenance.
The adhesive-coated string or tape material may be applied to substrates which are being processed through machinery at speeds up to 1000 feet per minute. At such application speeds, if the material slips from its application path, the material tends to rub against or wrap around the axle of the wheel intended to press the string or tape material against the substrate. Such stress creates pressure between the wheel and wheel frame, causing wheel binding and, ultimately, bearing and seal failures.
Thus, a need exists for a method and an applicator for efficiently applying an elongate continuous adhesive-coated reinforcing material to a substrate while overcoming the problems of prior applicators.
According to one aspect of the present invention, such a need is satisfied by heating a length of an elongate continuous reinforcing material having an adhesive intimately associated with and coating the material by moving the length through a heating unit from an inlet orifice to an outlet orifice while heating and softening the adhesive to its flow point. At least a portion of the adhesive flows and becomes disassociated from the reinforcing material when the reinforcing material exits the outlet orifice. The length of heated reinforcing material is pressed onto a substrate located proximate the outlet orifice, and the disassociated portion of the adhesive is intercepted and applied to the substrate. Thus, the applicator apparatus and the floor around it remain free from any adhesive that separates from the reinforcing material during the application process.
According to another aspect of the invention, a heat-transferrable elongate tube is inserted into the heating unit and a length of the reinforcing material is heated by moving it through the tube from a tube inlet to a tube outlet. The tube is removably located in the heating unit. Thus, if the tube becomes fouled with adhesive or other wood product debris from the manufacturing operation, the tube can simply be removed from the heater and replaced with a new tube without disassembling the applicator and with minimal applicator downtime.
According to another aspect of the invention, a length of the reinforcing material is moved through the heating unit including an elongate hollow outlet portion having a substantially constant radius of curvature. The reinforcing material enters the heating unit at an inlet orifice and exits the heating unit at an outlet orifice. The heated reinforcing material exits the heating unit on an arcuate path defined by the elongate hollow outlet portion through the outlet orifice. This arcuate path permits application of the heated reinforcing material to a substrate at the highest application speeds without applying significant and undue tension to the heated reinforcing material. In addition, the arcuate application path defined by the outlet portion encourages the softened adhesive to collect on that portion of the reinforcing material most distant from its area of contact with the elongate outlet portion, thus fostering an accumulation of adhesive at the eventual point of contact between reinforcing material and substrate for a secure bond between the two.
According to another aspect of the invention, the reinforcing material is heated and applied to the substrate by contacting the substrate and the length of reinforcing material with the rotatable peripheral surface of a wheel mounted for rotation on an axle while preventing the length of reinforcing material from contacting the axle and interfering with the rotation of the wheel. Thus, even if the reinforcing material becomes dislodged from its application path, it is prevented from interfering with the rotation of the wheel or damaging the axle.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.